A two-stroke engine that has a carbureted fuel system is, on the one hand, typically beset by the disadvantages of poor fuel economy and unacceptable exhaust emissions due in large part to co-mingling of a portion of an unburned fresh charge with exhaust products of the burnt charge being scavenged. On the other hand, this type of engine has the advantages of inherent simplicity and large power-to-weight ratio which make it particularly suitable for small and light engines, both for sea and land vehicles. Replacement of the carburetor by electronic fuel injection provides an exactly timed fuel feed that can dramatically reduce the above mentioned problems because it allows scavenging to be carried out before fuel is introduced into the combustion chamber.
Two important considerations for a fuel injection system are: (1) the timing (there is typically less than 5 milliseconds to inject the fuel), and (2) the quality of the injected fuel spray (which must be well atomized to reduce exhaust emissions). Various solutions have been proposed in attempts to meet both timing and spray objectives. Some combine pneumatic and hydraulic devices, with one system using air compressed at medium pressure (less than 10 bar) in order to atomize and carry into the combustion chamber fuel metered by a conventional low pressure injector into a prechamber. An additional special electrovalve is still needed in order to inject the atomized fuel/air mixture into the combustion chamber. Another solution (only used for small engines) uses air compressed at low pressure directly in the crankcase. The mixture is prepared in a Venturi-shaped prechamber, communicating with the cylinder head via a normal intake valve. The fuel is injected into the Venturi-shaped prechamber by a conventional low pressure injector and is atomized and carried into the combustion chamber when the intake valve opens.
The most promising system for automotive engines, from both exhaust emissions and vehicle driveability points of view, is electronic direct injection, to which the present invention is related. Such a solution, due to the very short allowable time for an injection, (about 3 msec max.), requires extremely fast valve actuation. The linear flow range must be very large (&gt;10 for 3 msec period) in order to encompass both minimum and maximum injected fuel requirements, and so this makes the minimum linear pulse width very short (&lt;0.5 ms). These requirements are very difficult to obtain due to the high injection pressure (also &gt;100 bar) needed for good fuel atomization. Additional difficulty is due to resistance to high temperature that a device directly faced to a combustion chamber must satisfy.
A fuel injector having the above characteristics can also be used for the direct injection of fuel into a four-stroke, spark-ignited engine and such an application is very promising due to the reduction of the pollutants that can be obtained.
It is evident from the previous considerations that a very powerful magnet is needed in order to actuate a valve with the required speed characteristics. It is well known that one of the most widely adopted solutions is a double axial gap magnet in which the magnetic flux path is formed by a stator composed of a central core surrounded by a solenoid and by an external housing and facing through two axial gaps, a large disk-shaped armature connected to a valve needle. When the solenoid is energized, the magnetic flux tries to reduce the gaps, thereby creating two axial magnetic forces.
Such a configuration has the following disadvantages:
(1) large mass of the armature, which increases the actuation times; PA1 (2) relatively small flexural stiffness, which can affect the vibrational behavior of the armature occurring after the contact with the movement stops, increasing the risk of bounces and therefore of extra fuel sprays; PA1 (3) large hydraulic resistance during the motion, due to the large cross section of the moving device; and PA1 (4) large sensitivity to geometric and positional tolerances. PA1 (1) small radial dimensions and mass of the armature for a very fast actuation; PA1 (2) high flexural stiffness of the armature, for low risk of bounces; PA1 (3) producability with standard machining techniques and easy assembly for a low cost; and PA1 (4) large axial magnetic forces. PA1 A) an external housing for providing a cavity; PA1 B) a solenoid coil disposed within said cavity; PA1 C) a magnetic circuit that conducts magnetic flux produced by energization of said solenoid coil and comprises, PA1 D) valve means that is operated by the longitudinal reciprocation of said ferromagnetic armature to open and close a flow path through the fuel injector; PA1 E) said stator comprising, PA1 F) an adjusting pin and an elastic biasing means disposed within said through-bore of said central magnetic pole such that said elastic biasing means is disposed between said adjusting pin and said armature, and said adjusting pin is positionable axially within said through-bore to set the force exerted by said elastic biasing means on said armature; PA1 G) said armature comprising, PA1 H) said armature being moved coaxially toward said stator upon energization of said solenoid coil; characterized in that: PA1 I) said upwardly facing ring-shaped end face of said armature and said downwardly facing ring-shaped end face of said central magnetic pole define a first axial gap between said stator and said armature: PA1 J) said further upwardly facing ring-shaped face of said armature and said ring-shaped margin of said downwardly facing surface of said pole washer surrounding the through-hole in said pole washer define a second axial gap between said stator and said armature such that said axial gaps tend to reduce upon energization of said solenoid coil; PA1 K) the maximum radial dimension of said armature from said longitudinal axis is substantially less than the radial dimension of said cavity within which said solenoid coil is disposed; and PA1 L) motion of said armature toward said stator is arrested by abutment of said upwardly facing ring-shaped face of said armature with said downwardly facing ring-shaped end face of said central magnetic pole and not by abutment of said further upwardly facing ring-shaped face of said armature with said ring-shaped margin of said downwardly facing surface of said pole washer surrounding the through-hole in said pole washer.
In fact, error in perpendicularity between the stator and the armature is amplified at the outer diameter of the armature. If contact of the armature with the stator is designed to be at or near the outside diameter of the armature, and there is significant error in perpendicularity of the armature, such contact will be restricted to a small surface area of the total circumference, which then becomes overstressed. If, on the contrary, the contact is designed to be closer to the center, a large residual gap must be left near the outside diameter, in order to avoid undesired contact, and that size of gap decreases the magnetic force during the actuation.
Another known solution that provides fast injector response is reduction of the eddy currents in the magnet. This is obtained by a laminated or powdered metal flux path, but this is often complicated and expensive, and generally needs large cross sections in order to have sufficient forces.
The objective of the present invention is to provide an electromagnetic valve for fuel injection offering the following advantages:
This objective, with a simple and economic approach, has been met by an embodiment comprising:
i) a ferromagnetic stator, and PA2 ii) a ferromagnetic armature that executes longitudinal reciprocation relative to said stator in response to the energization and de-energization of said solenoid coil; PA2 i) a ferromagnetic pole washer that is toward said armature and that has a through-hole at its center, and PA2 ii) a central magnetic pole comprising a through-bore extending coaxially within said housing to terminate in a downwardly facing ring-shaped end face proximate and coaxial with the through-hole in said pole washer; PA2 i) an upwardly facing ring-shaped face proximate and coaxially facing the downwardly facing ring-shaped end face of said central magnetic pole, and PA2 ii) a further upwardly facing ring-shaped face proximate and coaxially facing a ring-shaped margin of a downwardly facing surface of said pole washer surrounding the through-hole in said pole washer,
The invention is also characterized in that:
said pole washer is disposed perpendicular to said longitudinal axis of the fuel injector;
said further upwardly facing ring-shaped face of said armature is on a flange of said armature that is perpendicular to said longitudinal axis;
the magnetic circuit reluctance of said first axial gap is substantially equal to that of said second axial gap;
the magnetic circuit reluctance between said pole washer and said central magnetic pole at the region of said end face of said central magnetic pole and the through-hole in said pole washer is at least ten times greater than that at said first axial gap and at least ten times greater than that at said second axial gap;
thin layers of harder plating covering those portions of said central magnetic pole and said armature that abut each other for imparting increased impact resistance to those portions and for also guaranteeing a residual actual gap in the magnetic circuit when those two portions are in abutment with each other;
said pole washer can be either an integral part of said housing or a separate part that is assembled into said housing; and
the housing can be either ferromagnetic to form a portion of the stator or non-ferromagnetic to form no portion of the stator.
The features of the invention will become more apparent from the following detailed description of the presently preferred embodiments, with reference to the attached drawings which are representative examples of the invention.